Described herein are systems and methods for detecting ink jet print head defects when printing substantially clear, colorless inks. More in particular, described are systems and methods that detect print head defects by incorporating an ultraviolet (UV) or infrared (IR) sensitive material into the substantially clear ink, which UV or IR sensitive material is substantially invisible to the naked human eye under ambient light conditions, and thus does not alter the appearance of the substantially clear ink under such ambient light conditions, but which becomes visible, and thus machine readable or detectable, upon exposure to activating radiation (light) of the appropriate wavelength. The exposure of an image formed with a substantially clear ink printed through the ink jet head to the activating radiation can thus be used to allow for any defects in the ink jets of the head to be detected. Subsequent appropriate corrective action can then be taken.
Substantially clear overcoats are known and have several utilities, for example including in use as a filler ink in eliminating print ghosts, or as a patterned or full overcoat to adjust image gloss or to protect previously deposited color ink image, for example against scratches and the like. Such overcoats are known, and may be applied as either a blanket over an entire image or as a selectively applied and/or patterned overcoat.
Prior procedures for applying substantially clear overcoats have included flood coating techniques and the like for covering an entire image.
Although it may be desirable to be able to evaluate the accuracy of the application of the substantially clear ink to the substrate, for example in order to ensure appropriate coverage of the image by the substantially clear ink as a result of the ink jets of the ink jet head accurately applying the substantially clear ink to the substrate, such is difficult to accomplish when using a substantially clear ink because the ink is not readily detectable. As a result, defects such as clogging, misalignment and the like in the print head are not detectable when printing a substantially clear ink from the ink jet head.